Use putIfAbsent in InheritanceManager._reportError.

pkg/analyzer/lib/src/dart/resolver/inheritance_manager.dart

 // Copyright (c) 2016, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 import 'dart:collection';
 
 import 'package:analyzer/dart/ast/ast.dart';
 import 'package:analyzer/dart/ast/token.dart';
 import 'package:analyzer/dart/element/element.dart';
 import 'package:analyzer/dart/element/type.dart';
@@ skipping 651 matching lines @@
    * @param offset the offset of the location of the error
    * @param length the length of the location of the error
    * @param errorCode the error code to be associated with this error
    * @param arguments the arguments used to build the error message
    */
   void _reportError(
       ClassElement classElt, ErrorCode errorCode, List<Object> arguments) {
     if (ignoreErrors) {
       return;
     }
-    HashSet<AnalysisError> errorSet = _errorsInClassElement[classElt];
-    if (errorSet == null) {
-      errorSet = new HashSet<AnalysisError>();
-      _errorsInClassElement[classElt] = errorSet;
-    }
+    HashSet<AnalysisError> errorSet = _errorsInClassElement.putIfAbsent(
+        classElt, () => new HashSet<AnalysisError>());
     errorSet.add(new AnalysisError(classElt.source, classElt.nameOffset,
         classElt.nameLength, errorCode, arguments));
   }
 
   /**
    * Given the set of methods defined by classes above [classElt] in the class hierarchy,
    * apply the appropriate inheritance rules to determine those methods inherited by or overridden
    * by [classElt]. Also report static warnings
    * [StaticTypeWarningCode.INCONSISTENT_METHOD_INHERITANCE] and
    * [StaticWarningCode.INCONSISTENT_METHOD_INHERITANCE_GETTER_AND_METHOD] if appropriate.
@@ skipping 563 matching lines @@
   }
 
   /**
    * Initializes [keys] and [values].
    */
   void _initArrays(int initialCapacity) {
     _keys = new List<String>(initialCapacity);
     _values = new List<ExecutableElement>(initialCapacity);
   }
 }